Density is mass per unit volume, independent of gravity

Steps:

• Density is defined as ρ = m / V, where m is mass and V is volume.
• Mass m of the rock is invariant regardless of location.
• Volume V of the solid rock remains unchanged under different gravity.
• Thus, ρ stays the same on the other planet.

Why B is correct:

• Density depends only on mass and volume, per the standard definition, unaffected by gravitational acceleration g.

Why the others are wrong:

• A: Halving density ignores that mass and volume are fixed, not scaled by g.
• C: Doubling density wrongly assumes g directly multiplies density.
• D: Quadrupling density has no basis, as g's square doesn't apply to density.

Final answer: B